The present invention pertains to the use of a borehole, and associated electrical conductors installed as part of a well completion, as a source antenna for geophysical applications.
The embodiments described herein relate generally to soundings within the earth based upon electrical fields. As used herein, “earth” or “Earth” generally refers to any region in which a borehole may be located including, for example, the lithosphere. Electromagnetic (EM) geophysical surveys probe electrical resistivity, or equivalently, conductivity, in the earth as a function of depth. Typical targets of interest include, without limitation, ore bodies, hydrocarbons, water/steam, proppants, hydraulic fracture (or fracking) fluids, salts, CO2, and other substances injected into the ground, drill hazards such as karsts and hydrates, tracers, markers, permafrost/frozen tundra, hot rock and other geothermal indicators, and environmental pollutants. Since the resistivities of such targets and the surrounding medium may be quite dissimilar, the targets may be discriminated by measuring their subsurface resistivities when subjected to an electromagnetic field. Using this methodology, the depth, thickness, and lateral extent of materials of interest may be determined or monitored.
The source of the EM field used in a geophysical survey may originate in the natural environment or be manmade. If manmade, the source may produce a primarily magnetic or electric field that varies in time, and this primary field can produce a secondary field in the conducting earth. For example, an electric field can produce electric currents in the earth that have an associated magnetic field, and a time varying magnetic field induces electric currents that result in an electric field. The electrical properties of the earth and rate of change of the field determine the relative magnitudes of the secondary and primary fields. The combination of primary and secondary fields results in combined electromagnetic interaction with the earth even for a source arranged to produce solely an electrical or magnetic field.
The distribution of electric current flow produced by an EM source is determined by the three-dimensional (3D) resistivity distribution within the earth. The electric field measured at the surface or at depth within a borehole can be used to infer the 3D resistivity variation over the region where significant current is flowing. The current is typically measured by a suitably calibrated array of electric or magnetic field sensors. The resulting 3D resistivity variation can be used to project the distribution of ores, hydrocarbons, or water within the measured volume.
While the majority of EM geophysical surveys are performed with sensors and EM sources on the surface of the earth, a borehole can provide physical access to the subsurface. Measurement of the electric or magnetic field within a borehole can be related to the electric or magnetic field in the earth around the borehole or the fields that would exist in the earth in the absence of the borehole. Similarly, connecting an electric field or magnetic field source to the earth via a borehole provides a way to produce fields within the earth at desired depths without the attenuation and uncertainties that may result if the source fields originate from a source at the surface of the earth. A particularly beneficial configuration of a borehole EM source is an electrode situated at the top or bottom of a borehole casing, and in electrical contact with that casing, and one or preferably a group or suite of source electrodes at the surface and which may be approximately arranged in a ring approximately centered on the borehole. In this case, significant electric currents in the ground are caused to flow at depth out to a radial distance from the borehole to the surface. Similarly, it is not required that a cased well be utilized, since the electrode can be grounded directly to the earth in an open hole which may or may not contain steel casing at all, resulting in an equally beneficial current flow outward from the borehole at reservoir depth.
A new commercial sounding configuration is the Borehole to Surface EM (BSEM) method. FIG. 1 illustrates the current practice comprising an electrode at depth within a borehole, termed the source electrode, and an electrode at the ground surface disposed near to the well that acts as a counter electrode. A transmitter produces a voltage that induces an electric current to flow between the source and counter electrodes. However, the direction of current flow is in general oscillatory, and it is equally true to say the current flows into the ground from the counter electrodes and out via the source. A surface array of receivers measures the EM fields induced by the source.
An advance described in a recently filed International Patent Application No. PCT/US2012/039010 titled “System and Method to Measure or Generate an Electrical Field Downhole” by Hibbs and Glezer (incorporated herein by reference) involves locating a number of counter electrodes at a distance from the well comparable to the depth of the source electrode, and at least not less than 10% of the borehole depth. As illustrated in FIG. 2 of the present application, the subsurface current is forced to flow laterally through the ground (i.e., orthogonal to a vertical borehole) by a distance at least equal to the radial distance between the source and counter electrodes. This configuration increases the current flowing in the ground at formation depth and at large lateral offset from the borehole. Similarly, the placement of the return electrode when transmitting current along a horizontal well can be used to preferentially alter the current flow to focus on a specific region in the subsurface (e.g., placing the return electrode above the toe of a horizontal well to increase current flow out of the toe of the well casing).
A disadvantage of the BSEM method is that the borehole must be opened and a wireline is required to lower the source electrode to the desired depth. With this in mind, it has also been proposed, particularly in International Patent Application No. PCT/US2013/058158 titled “System and Method to Induce an Electromagnetic Field Within the Earth” by Hibbs and Morrison (incorporated herein by reference), to not employ a source electrode within the casing at depth, but rather drive the entire casing of the borehole at the desired voltage, V, by making an electrical connection at the top of the casing. Such an arrangement is represented in FIG. 3 of the present application. The top connection can also be implemented by an electrode in the ground near to the casing so that current flows through the earth from the near electrode to the top of the casing. For convenience, an EM source configuration comprised of a conducting well casing and a suite of surface counter electrodes of this type can be termed a Depth to Surface EM (DSEM) source.
In the BSEM and DSEM source configurations shown in FIGS. 1-3, a significant fraction of the total transmitted current flows in the casing. However, even for a uniform casing, the amount of electric current in the casing is not constant along its length. In the configuration shown in FIG. 1, electric current is emitted from the casing into the earth at the bottom of the casing, where the internal source electrode is located, and also emitted into the earth along the entire length of the casing. In the configuration shown in FIG. 2, electric current flows into the earth from the source grounding point at the bottom of the casing and also from along the entire length of the casing. When contact is made by the source to the top of the casing or near the top of the casing as in FIG. 3, current flows down the casing along its entire length, and is radially distributed along the length of the casing into the surrounding earth.
Historically, the presence of conducting casings in boreholes has been considered a problem for surface EM surveys (for which all equipment is deployed at the ground surface), and such surveys have been arranged to avoid placing sources or receivers close to casings. For the recently introduced BSEM method illustrated in FIG. 1, the majority of commercial surveys have been conducted in uncased wells, thereby eliminating the question of current flow in the casing. However, the great majority of boreholes are completed with electrically conducting casings. Furthermore, the DSEM configuration shown in FIG. 3 requires a conducting casing. Therefore, it is of significant practical and commercial benefit to be able to utilize EM data collected via a source that utilizes a cased well.
Many studies of EM geophysical surveys discuss an electrical or magnetic source in a uniform or infinite vertical steel pipe in a layered or cylindrically symmetrical media. This has proven useful, particularly in logging applications, where the targets of the survey are the layers adjacent to the wellbore. Unfortunately, this is an insufficient approximation to real world applications in which wells deviate from the vertical or even extend horizontally and where carbon steel pipe and cementation can vary in physical properties and dimensions throughout the wellbore. In these cases, the source function can be quite complex and describing the generated formation currents is more involved. With this in mind, the workflow described below allows this issue to be addressed in a more general form where the casing is replaced by an equivalent array of electrical sources through a 4-step process. These sources can be applied to 1D, 2D or 3D geophysical problems where the casing geometry may be vertical, deviated, horizontal, fishbone, or similar borehole configurations.